Chemical studies on soil humic acids

Abstract

Forty humic acid (HA) samples and several related substances were reduced with Na₂S₂O₄ and NaBH₄ in order to investigate the effect of carbonyl groups including quinone on the ultraviolet and visible absorption spectra of HAs. Difference spectra of Rp and B type HAs as well as lignin due to NaBH₄ reduction had a peak at 320 run, while those of A type HAs had a peak and a shoulder at different wavelengths. The difference absorbance of A type HAs in the visible range was also higher than that of Rp and B type HAs. There were very significant positive linear correlations between the difference absorbance due to NaBH, reduction and carbonyl group contents of HAs. The absorbance of the highly humified HAs at 600 run increased in 0.1 n NaOH but decreased in the phosphate buffer at pH 7 by the reduction with Na₂S₂O₄. Decreasing rates in both cases decreased with increasing degree of humification. It was presumed that the influence of ketonic and aldehydic carbonyl groups on the visible absorption was small and that of quinones was large for the low humified HAs, P_+-+++ type HAs and the two artificial HAs prepared from hydroquinone and alucose. Though the influence of quinones also increased, that of aliphatic ketone and aldehyde on the visible absorption increased with increasing degree of humification.